If you are having e-mail, contact, or calendar issues with Entourage in the shared exchange environment, it could be a bad cache file. It used to be that the only thing to do was to remove the entire profile and recreate it, but thanks to Microsofts incredible wisdom, they somewhere along the way added the "Empty Cache" option to Entourage 2004. Now, if you are seeing glitches somewhere, i.e. hung up email messages, messages not getting through but in your webmail or blackberry, or calendar items not syncing, this could be your cure all. It is quicker and more efficient than resetting cache in Outlook, which requires multiple steps and a good chunk of time. To clear your cache in Entourage, simply highlight the parent folder of your exchange mailbox (it will work in any folder, but i am unsure as to whether this clears just that folder or everything, so I am suggesting doing a blanket clear here) then click the Edit menu from Entourage and then click "Clear Cache." This will pop up a window that says "Emptying the cache will remove and re-download all items within this account. Unsynched changes, links, projects, categories, and action indicators for messages will be lost. Are you sure you want to do this?" This means that anything on the machine that is not on the server will be lost. If you can back any of this up, now is the time to do that, otherwise be prepared to re-do it. Once you are sure you are safe, click "Empty Cache" and you should see things re-syncing with the server without all the effort of removing your profile and re-creating it!
7/23/2008:
Ok, so I have come to discover that my machine is one of only a few with this "miracle cure" on it. But, fear not, Entourage-heads, I have a solution! If you do not have the "Empty Cache" option in your Entourage, here is a fix for 2004 AND 2008. NICE! If you go to your main folder (the one you gave some cool name to, like "My Account" or "Work Mail") and ctrl+click on it, you will see an option for "Folder Properties". Under "General Tab" you should se a sections that says... wait for it... "Empty Cache". Just hit the "Empty" button and you are ready to go!